Corrosion inhibition of titanium-base materials by fuming nitric acid



United States Patent CORROSION INHIBITION OF T ITANIUM-BASE MATERIALS BY FUMING NITRIC ACID Howard B. Bomberger, East Liverpool, Ohio, assignor, by mesne assignments, to Crucible Steel Company of America, Flemington, N. .L, a corporation of New Jersey No Drawing. Application July 19, 1956 Serial N0. 598,743

7 Claims. (Cl. 23-1 57) 'This invention relates to fuming nitric acid compositions with decreased ability'to cause corrosion and stress corrosion cracking of titanium and titanium base materials. In another aspect, it relates to methods ofinhibiting corrosion and stress corrosion cracking of titanium base materials by red fuming nitric acid.

Titanium is well known for its unusually high resistance to corrosion by most chemicals. Likewise, the titanium base materials, i. e., titanium alloys containing a major portion of titanium, are also'quite resistant to corrosion. The term titanium'bas'e material as employed herein is intended to include not only titanium alloys but also the unalloy'ed metal. These materials originally appeared ideally suited as containers for nitric acid solutions of all concentrations, and considerableinterest developed in the "use of titanium base components in the handling of nitric acid of very high concentration, more particularly, red fuming nitric acid. Early studies indicated that these materials possessed'excellent corrosion resisting properties in the presence of this" acid. However, later more exhaustive investigations detected instances where the corrosion rate was abnormally high. That isto say, the usual low rate of lessthan 0.1 mil penetration per year, which is considered acceptable, was exceeded many times. These tests also establishedthat the titanium alloys tend to corrode at amuch higher rate in the presence of red fuming nitric acid than unalloyed titanium.

Early corrosion studies of titanium and itsa-lloys in the presence of red fuming nitric acid indicated that corrosion is accomplished by formation on the metal surface of an adherent, dark, generally black, corrosion product which is pyrophoric. This substance can be detonated in an oxidizing environment by shock, heating, or by an electric spark. When removed from the surface of the metal, it can be similarly detonated. Its sensitivity to impact is approximately equal -'to that of nitroglycerine and mercury fulminate. The-black corrosion product 'has been identified as very finely divided titanium metal, 'and is the result of extensive intergranular corrosion. Its sensitivity has resulted in atleast one fatalaccidenn' and' has caused considerable reluctance to employ light weight titanium and titanium alloysas containers for red turning nitric 'acid.

In addition to 'formation of the pyrophoric s'ri'r'face coating,"the"metal has been found to undergo considerable stress corrosion cracking in the presence 'of the fuming acid. This cracking is particularlyevident arou'nd number stamps and other identifying marks and notches, as well as at the sheared edges of test samples. The titanium alloys are considerably more susceptible to stress corrosion cracking than is commercially pure titanium metal.

.While considerable work has been done identifying the corrosion product, investigating the susceptibility of titaniumbase materials to corrosion cracking, measuring corrosion rate and correlating corrosion and cracking rates with the N and water concentration of the fuming to corrode these materials.

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'2 nitric acid, as yet there has been nothing publishedregarding methods or compositions for inhibiting or substantially eliminating the corrosion.

fuming nitric acid solution which would not/corrode titanium and its alloys would behighly desirable.

The present invention provides two methods for 'substantially eliminating serious corrosion'and stress 'corrosion cracking of titanium base materials by fuming nitric acid in sealed containers, as well as ajr'nodified red fuming nitric acid composition of substantially reduced"ability The first of these methods,'referred to as the oxygen method, comprises providing a vapor space above there'd fuming nitric acid in the sealed titanium base vessel maintaining the concentration of oxygen in this vapor zone at least equal to 5% by volume. In the case of titanium base acid containers open to the atmosphere, no precautions need be taken, since there is sufficient oxygen in the air to'provide the required oxygen concentration at the liquid-vapor interface. The titanium bas'e 'acid containers find their widest application where sealed.

employed, I prefer commercialoxygenfor best results.

Although a minimum of 5% oxygen by volume at the liquid-vapor interface is sufficient to arrest. and satisfactorily control corrosion, the oxygen concentrationina'y vary widely above this figure even to as much 'as lO0% oxygen in the vapor zone It is preferable to Tfiush the vapor zoneperiodically with oxygen.in 'high concentration, since flushingpmaythen be at relatively widely spaced intervals as distinguished from the frequency necessary when air containing only about 20% oxygen is employed. 'Of course, the required frequency bffflrish- 'ing, whether with oxygen or air, will de'pendf'upon the relative volumes'of the vapor zone and 'ac'idjand the area of the liquid-vapor interface. In'tests which I have conducted, the volume of the vapor zone was about 25% of the total container volume, and flushing was effected daily with a volume of oxygen at atmospheric pressure about equal to the total container volume. However, these tests were :no't carried out in order to formulate a flushing frequency in hours or days, and it should be remembered that the controlling factor is the minimum oxygen concentration at'the interface, which should not be allowed to fall below 5%.

Whilethe'mechanisin by which oxygen prevents corrosion is not yet fully apparent, and I do not wish to be limited or otherwise prejudiced by an explanation, there is reason to believe that small amounts of N0 and HNO are reduced to NO and 'HNO in the metal 7 Accordingly,- methods of inhibiting titanium base corrosion, and/or a-modified 'filling of the container. hibitor is added and dissolved or is otherwise present in corrosion and cracking of titanium base materials.

3 densome technique, and means for flushing the contain ers may not always be available. The other method by which I am able to substantially eliminate corrosion overcomes the necessity for periodically changing the atmosphere in the container, and also eliminates the necessity for the vapor zone thus permitting complete By this second method, an inthe fuming nitric acid.

I have found that certain metal ions, in their highest degree of oxidation when present in very small concentration in the fuming nitric acid, substantially reduce More particularly, I have found that the presence of cupric, chromic, ferric, manganic and stannic ions as well as mixtures of one or more of these ions in the fuming nitric acid effectively inhibits corrosion. The degree of corrosion and cracking inhibition realized is entirely satisfactory, and when cupric ions are employed, inhibition is substantially the same as that afforded by the oxygen flushing method. In accordance with the present invention, these ions are added to red fuming nitric acid in the form of salts in amounts sufiicient to provide a metal salt concentration at least equal to about 0.01% by weight of the acid. In some instances, a smaller amount is effective. For example, 0.008% by weight copper sulfate is adequate. This latter figure corresponds to a cupric ion concentration in the acid of about 0.003% by weight. However, for good control and long-time applications, an excess over the above 0.01% minimum can can be employed without harm, and is desirable in order to prevent depletion of the inhibitor. Amounts in excess of about 2% by weight of the salt are unnecessary, and this figure appears to be a practical maximum for longtime storage conditions. About 0.1% salt is recommended for most conditions.

The metal ions employed are in their highest degree of oxidation. This is important since the metal ions in 'a lower state of oxidation are unstable in and are oxidized by the acid, which results in simultaneous reduction of some of the N and HNO to NO and HNO respectively. It is my belief that this reduction should be avoided since the reduction products, NO and I-INO appear to accelerate corrosion. For this reason, I do not introduce the metals as such or salts thereof in which the metal is not in its highest degree of oxidation.

Cupric, chromic, ferric, manganic and stannic salts,

such as their oxides, bromides, iodides, carbonates, sulrate. Container 3 was open to the atmosphere.

4 ciently low'so as to have no effect upon corrosion rates. To a lesser extent, chloride ions also appear to interfere with the desired result, and particularly in combination with copper. On the other hand, ferric chloride is a satisfactory inhibitor. Thus, it can be said that when cupric ions are employed as the inhibitor, the acid should be substantially free of chloride ions. In addition to the binary salts indicated, complex salts, which are generally less stable, e. g., dichromates and permanganates, may be used successfully.

Several series of tests were conducted in order to es tablish the corrosion rate and extent of stress corrosion cracking in titanium base materials by red fuming nitric acid. The tests illustrate the effect of the metal ion inhibitors, and also the effectiveness of air and oxygen at the liquid-vapor interface.

EXAMPLE 1 In the first of these tests, samples of titanium metal and a titanium-7% manganese alloy both in the asreceived and 20% cold rolled condition were immersed in 400 ml. of red fuming nitric acid HNO 20% N0 in 570 ml. polyethylene bottles. The metal samples measured 0.050" x 0.5" x 4". One of the containers was left open to the atmosphere, while the remaining four were sealed by means of a softened parafiin block which was held in place by a five pound weight. Each of the bottles contained a titanium metal sample in the as-received and in the 20% cold rolled condition as well as two similar samples of the titaniummanganese alloy. Samples remained in the acid for 28 days at room temperature. No inhibitor was added to the acid in container 1. Container 2 contained in addition to the red fuming nitric acid 1.5% by weight of water in order to illustrate its effect upon corrosion Container 4 was sealed and the vapor space above the fuming nitric acid, amounting to about /4 the volume of the container, was flushed daily with approximately 570 ml. of oxygen at atmospheric pressure. The oxygen line to this container was destroyed by acid vapors after three weeks, so oxygen flushing was accomplished for only of the test period. To container 5 there was added 5 grams of anhydrous copper sulfate equivalent to 0.32% by weight cupric ions.

The following table reflects the corrosion rates of the metal samples in mils per year after 28 days, the appearance of the samples following the test, and a notation regarding the stress corrosion cracking, if any, profates, nitrates and phosphates may be employed with duced by the test.

Table l Corrosion Rates In Mils per Year Container Conditions ('28 day No. test) Titanium Ti-7% Mn Remarks As Recd. 20% O. B. As Recd. 20% O. R.

1 Sealed 1. 32 1.80 3.04 3.00 Heavy staining and cracking. 2 Sealed, 1.5% H10-.. 7.43 7.15 11.23 10.72 Heavsi'dstaining, etching and CF80 Hg. 3 Open .018 0.00 0.00 .014 No staining or evidence oi corrosion or cracking. 4 Sealed 0; flushing .056 .028 .023 .202 Very kslight: staining. No

. crac ms. 5 Sealed 0.32% Ou++ .009 .005 0.000 .014 No staining or cracking. No

as 01150 evidence of attack.

red fuming nitric acid, since it appears that the metal ion and not the anion of the salt is the effective in-- hibiting agent. However, I have found that fluorides of these metals, and fluoride ions in general, seem to interfere with inhibition. Accordingly, the fuming nitric. acid is maintained substantially free of fluoride ions, that is, fluoride ions are absent or their concentration. is 1 01 Container 1 which was sealed is taken as the standard 70 for comparison, and it will be observed that an addition of only 1.5 by weight of water to the red fuming nitric .acid increases the rate ofcorrosion of both the titanium and the titanium alloy from three to four times. The samples in container 3, which was open to the atmosphere and in hiFh the oxygen concentration at the "goddess 2'5 ui a n rtass as i s an I t ata th a r r t j r e r ly flo essin hei'sa esf titwnium djapproxi a y 30 tim. les n thesase f the alloy than the samples in sealed container f 1. The samples in sealed. container 4 to which oxygen was intro duced daily for three of the tour we ks f'the test, cor-j roded ata rate somewhat greater than those in; container 3, but this can be accounted for in part by thefhreak: down which occurred. Nevertheless, the low rateio'f corrosion observed is satisfactory, and the results clearly indicate that'oxygen, was the element instrumental in inhibiting attack of the samples. Examination of the cor: ro sion rate and remarks in connection with container 2, which contained the small amount of water clearly indicates that moisture is in no way'beneficial as an inhibitor and that it is oxygen and not moisture contained in the air which inhibited attack, in container 3.

The 0.32% cupric ion concentration in container 5 effectively inhibited both corrosion and stress corrosion cracking, and was as effective as an open container. While the mechanism of inhibition in this case is not clearly understood, it is apparently different from the action of oxygen. Quite possibly, corrosion is prevented by. adsorption of cupric ions on the surface of the titanium base material.

EXAMPLEv 2 The second series of tests was similar to-th'ose of Example 1, but carried out over a period of only seven days. The inhibiting eifect of manganic, ferric, and chror S inh bit ng fie' t o oxyge of t a r- Sealed o ain rs 3, 4 andS contained 5 grams of manganese dioxide, ferric chloride and chromic'oxide, respectively. While it' is apparent that manganic and ferric ions are not as eiiec tive as the chromic ion, each of these three ions eifec tively reduced corrosion rates, particularly on the titaniufn metal samples. The rate of reduction is somewhat less on the alloy samples; Of the three, in the ionic con centration employed, chromic ions are clearly the most effective on titanium metal and ferric ions are bestfor the titanium-manganese alloy.

} EXAMPLE 3 The third series of tests was conducted in order to ob: serve the efiect of the degree of oxidation of the copper ions upon ability to inhibit corrosion. The tests were'in general carried out as described in Example 1. How: ever, the samples were of somewhat diiferent size, ,name- 1y, 0.035" x 0.3" x 4", and the duration of the test was 31 days. Each of the bottles contained 400 ml. of solution and an air vapor zone of 150 ml. All of the containers were tightlysealcd by placing a /2 cap of paraffin over the top and weighing it with approximately 2 pounds. The results of these tests appear in Table III. The small corrosion rates recorded could in isolated. instances be due to weighing errors, since a corrosion rate of 0.005 mil per year is equivalent to a weight loss of only 0.1 milligram. Consequently, where there was little weight loss, surface appearances, such as staining and cracking, are a more accurate indication of corrosion.

Table III Corrosion Rate in Mils per Year Container Conditions (Contain No." .ers Sealed 31 day test) -T-i Metal Ti7% Mn Remarks As Recd. 20% O. B. As Recd. 20% C. R.

RFNA only .092 .054 .097 .070 Heavy staining and corrosion. RFNA 0.003% 005 .016 054 .087 N0 Staining.

Cu As CuSOr RFNA 0.06 .032 .022 .24 D0.

Cu++ as 011504. RFNA .06% Cu'' .033 .038 .37 No staining. Cracking on all as CuOl. but as-received titanium. RFN;:A1+ 0.06% Cu .016 .022 .31 79 Do.

me a

mic ions was observed. Each bottle contained samples of both titanium and titanium-7% manganese in the asreceived condition and titanium in 20% cold rolled concupric ions.

dition. The corrosion rates and extent of cracking are noted in Table II.

No visible surface changes occurred in the titanium metal or the alloy in containers 2 and 3 which contained The presence of copper in the form of cuprous chloride in container 4 in amount equivalent to the cupric ion concentration of container 3 prevented stain- Table II Corrosion Rates in Mils per Year Container No. Conditions (7 day test) Titanium Remarks Ti-7% Mn As Rec'd. As Recd. 20% C. R.

1 Sealed 3. 00 1.21 11.25 Heavy staining with corrosion produet covering the surface. Extensive cracks developed at stampings and rolled edges.

2 Open 0.070 0.090 0.090 No tarnishing or corrosion. Two minute cracks developed on the O. R. Ti sample several weeks after tests.

Sealed 5 gins. MnOa 0.95 0.10 7.5 Slight staining. Cracks developed .5 Mn" at stampings and rolled edges. Sealed -l- 5 gms. FeOl; 0.75 1.25 2.85 Slight staining. Cracks developed (0.27% Fe+ at rolled edges only. Sealed 5 gins. CiOa 0.07 0.107 6.32 Very slight staining. Cracks de- (0.4% Cr). vgloped at stampings and rolled e ges.

Container 1 illustrates uninhibited corrosion rates,

ing on all of the samples, but stress corrosion cracking 00- while container 2, which was open, again illustrates the curred in all samples except the as-received titanium metal. Since dilute hydrochloric acid has been found to cause rapid stress corrosion cracking under certain conditions, it is believed that cracking of the samples in container 4 was caused for the most part by the chloride ion. The presence of cupric ions following oxidation definitely had an effect on controlling staining. Examina- 'tion of the samples from container 5 containing metallic copper revealed nostaining, but all except the as-received titanium metal developed cracks. Since metallic copper reduces nitric acid, reduction products of red fuming nitric acid such as NO and HNO are formed. These reduction products are regarded as undesirable in controlling corrosion, and it is believed that metallic copper in this manner defeats the purpose of the present invention.

The present invention thus provides needed methods for inhibiting corrosion of titanium base materials by fuming nitric acid, and these materials need no longer be con sidered unsafe in the handling and storage of this acid. Of the metallic ions specified herein which may be effec' tively employed, cupric ions are generally preferred.

What is claimed is:

l. A method of inhibiting the corrosion of titanium base material by red fuming nitric acid, which comprises incorporating with said fuming nitric acid at least about 0.01% by weight of a salt of a metal in its highest degree of oxidation selected from the group consisting of copper, chromium, iron, manganese, tin and mixtures thereof, and maintaining said acid substantially free of fluoride ions. 1

2. A method of inhibiting the corrosion of titanium and titanium-containing alloys by red fuming nitric acid, which comprises incorporating with said acid a cupric salt in amount sufiicient to provide a concentration therein of at least about 0.003% by weight cupric ions, and maintaining said acid substantially free of fluoride ions.

3. A method of inhibiting corrosion and stress corrosion cracking of titanium base material by red fuming nitric acid, which comprises incorporating with said acid a cupr'ic salt having an anion selected from the group consisting of sulfate, nitrate, oxide, phosphate, iodide and bromide in amount sufiicient to provide a cupric ion concentration in said acid of at least about 0.003% by weight, and maintaining said acid substantially free of fluoride ions.

4. The method as set forth in claim 3, wherein the cupric salt is cupric nitrate, and the weight of salt added is at least about 0.01% by weight of the fuming nitric acid.

5. A method of inhibiting corrosion and stress corro sion cracking of titanium base material by red fuming nitric acid, which comprises incorporating with said acid cupric sulfate in amounts suflicient to provide a cupric ion concentration in said acid of at least about 0.003% by weight, and maintaining said acid substantially free of fluoride ions.

6. The method as set forth in claim 1 wherein the metal salt is added in amounts between about 0.01 and 2% by weight of the red fuming nitric acid.

7. A method of inhibiting the corrosion of titanium and titanium base alloys by red fuming nitric acid, which comprises incorporating with said acid at least about 0.01% by weight ferric chloride, and maintaining said acid substantially free of fluoride ions.

, References Cited in the file of this patent 

1. A METHOD OF INHIBITING THE CORROSION OF TITANIUM BASE MATERIAL BY RED FUMING NITRIC ACID, WHICH COMPRISES INCORPORATING WITH SAID FUMING NITRIC ACID AT LEAST ABOUT 0.01% BY WEIGHT OF A SALT OF A METAL IN ITS HIGHEST DEGREE OF OXIDATION SELECTED FROM THE GROUP CONSISTING OF COPPER, CHROMIUM, IRON, MANGANESE, TIN AND MIXTURES THEREOF, AND MAINTAINING SAID ACID SUBSTANTIALLY FREE OF FLUORIDE IONS. 